A three-term contingency is one in which behavioral acts in animals and humans is broken down into three key parts. The famous behavioral scientist B.F. Skinner coined the term and theorized the concept. The three parts of the three-term contingency are discriminative stimulus, operant response, and reinforcer/punisher. This three part contingency is actually fundamental to the entire study of operant conditioning.
Operant conditioning which is also sometimes called instrumental conditioning is a method of learning that relies on rewards and punishment for certain behaviors. The outcome is that an association is made between a behavior and the reward or punishment that goes along with it.